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Lidar and Triple-Wavelength Doppler Radar Measurements of the Melting Layer:

A Revised Model for Dark and Bright Band Phenomena

Kenneth Sassen1, James R. Campbell2, Pavlos Kollias3, Matthew Shupe4, Christopher

Williams5, and Jiang Zhu1

1. Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK

2. Science Systems and Applications, Inc., Greenbelt, MD

3. Rosenstiel School of Marine and Atmospheric Science, University of Miami, FL

4. NOAA Environmental Technology Laboratory, Boulder, CO

5. University of Colorado Cooperative Institute for Research in Environmental Sciences

and NOAA Aeronomy Labatory, Boulder, CO

Abstract. During a recent field campaign in southern Florida, rain showers were probed

by a 0.523 µm lidar and three (0.32-, 0.86-, and 10.6-cm) Doppler radars. The full range

of backscattering phenomena was observed in the melting region, including the lidar and

radar dark and bright bands. In contrast to the 10.6-cm radar data, only intermittent

evidence is found at 0.86 cm for the radar bright band, and no clear examples are seen at

0.32 cm. Analysis reveals that the 0.86-cm radar dark band is due to non-Rayleigh

scattering effects in large water-coated snowflakes. The lidar dark band exclusively

involves mixed-phase particles and is centered where severely melted snowflakes

collapse into raindrops, where spherical particle backscattering mechanisms first come

into prominence. Index terms: 0320, 0394, 3354, 3360.

1. Introduction

Much of our planets precipitation originates as snow far above the surface of the

Earth. Indeed, understanding the hydrological cycle requires a good working knowledge

of the production of the ice particles that contribute to snow and rain under a variety of

meteorological conditions. As a consequence of our knowledge of the physics of

precipitation formation, rain from melting snow is indicated to be the dominant process in

temperate zones, and also likely plays an important role in deep convective activity

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worldwide. Although the microphysical processes describing the transition of

snowflakes to raindrops are by now well understood, the corresponding effects on the

propagation of laser light and microwaves would appear to require more research. Now

that precipitating clouds are coming under scrutiny from Earth-orbiting radar systems,

improving our understanding of the scattering and attenuation of microwaves in the

melting layer is particularly warranted [Simpson et al., 1996; Stephens et al., 2003].

The most widely recognized remote sensing feature observed during the melting

of snowflakes is the radar bright band. Named after the appearance of the narrow layer

of strong signals on the oscilloscope displays of World War II vintage radars, it was not

long before the main causes of the bright band were identified [for a review see Battan,

1973]. Because of the differences in the dielectric constants between water and ice

particles, ice produces much weaker backscattering and attenuation in the Rayleigh

regime, such that radar returns in the rain are strongly enhanced despite the larger sizes of

the low-density (ice plus air mixture) snowflakes [Meneghini and Liao, 2000] and the

fact that the concentration of raindrops declines significantly because their fallspeeds are

much greater than the snowflakes they are derived from. A major factor contributing to

the radar bright band is a consequence of the manner in which snowflakes melt—the ice

surfaces become coated with liquid to a sufficient depth to scatter essentially like

equivalent-sized water particles. These wet snowflakes are nonspherical and still

relatively large, which generates strong microwave backscattering. Other factors that

may come into play involve the aggregation/coalescence or breakup of snowflakes and

raindrops, which, because of the diameter-to-the-sixth D6 power law of Rayleigh

scattering, can have noticeable effects on radar signals. At millimeter wavelengths a

bright band effect is typically absent, and a radar dark band has even been reported for

W-band radars near the expected bright band position [Lhermitte, 1988]. There is an

analog of the bright band with lidar, but the lidar bright band owes its existence to the

strong backscattering coupled with the overwhelming (i.e., exponential) attenuation rate

in the snowfall surrounding the freezing level, which can create a feature resembling a

bright band on an oscilloscope display [Sassen, 1977a].

The lidar dark band is a recently recognized curiosity of remote-sensing melting

layer features. Although long serendipitously captured in lidar returns from precipitation,

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its meaning and significance were concealed essentially because of a lack of prolonged

data collection in rainfall: lidar systems needed to be shielded from precipitation, and

other (optically less-dense) targets were favored. It was not until relatively recently that

Sassen and Chen [1995] comprehensively studied this phenomenon and gave it a name.

More recent observations have been reported in Demoz et al. [2000] and Roy and

Bissonnette [2001]. The term lidar dark band, in obvious contrast to the radar bright

band, delineates its quintessential property. It is a backscatter intensity minimum that

occurs in the melting layer, apparently not far in height from the location of the radar

bright band. Based on an analysis of coordinated aircraft, polarization lidar, and W-band

(0.32 cm) Doppler radar measurements, it was concluded by Sassen and Chen [1995] that

this “remarkably narrow and consistent feature” corresponded to a stage of snowflake

melting that produced “inhomogeneous ice-containing raindrops formed by the structural

collapse of severely melted snowflakes”. According to laboratory studies of melting

drops, the presence of an ice mass within a drop suppresses a major backscattering

mechanism in spheres—the axial ray bundle reflected off the far face of the drop [Sassen,

1977b]. The increase in backscattering aloft of the dark band center could be attributed

to the steady increase in low-density snowflake cross sectional areas in going from wet to

dry snowflakes [Sassen, 1977a].

Unfortunately, it has yet to be determined how representative the Sassen and

Chen [1995] findings are, particularly with regard to the melting layer temperature

structure, the precipitation rate and mechanism, and how measurements at other radar

wavelengths would have compared. The same can be said of the representativeness of

the available W-band radar dark band case studies. These issues are addressed in the

current study based on a unique ensemble of remote sensors.

2. The Dataset

The Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area

Cirrus Experiment (CRYSTAL-FACE) field campaign, which was carried out during July

2002 in the southern Florida region, was designed to research subtropical thunderstorms

and the cirrus clouds derived from their anvils. In addition to six project aircraft, three

surface sites were equipped with various ensembles of remote sensing systems to obtain

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more continuous atmospheric observations and serve as hubs for the aircraft operations.

The eastern site at the Kendall-Tamiami Executive Airport (~25 km southwest of Miami)

was uniquely equipped with three Doppler radars and a radiation measurement suite that

included a near-continuously operated eye-safe lidar (see Table 1 for remote sensor

specifications). Data were also collected by a Joss-Waldvogel disdrometer located at this

site to obtain high (1-min) resolution rainfall rate measurements. It should be noted,

however, that surface rain measurements can differ greatly from conditions aloft in the

melting region (~3.5 to 4.5 km above sea level in this case) because of temporal

variations in the showers. Figure 1 shows an aerial photograph of the major instruments

as deployed at the field site.

The micropulse (0.523 µm) lidar [MPL, Spinhirne, 1993] is a compact, eye-safe

device, which is being increasingly utilized worldwide at ground-based observing sites

for unattended cloud and aerosol observations. Eye-safety is achieved by using a rapidly-

pulsed (2.5 KHz), low-powered (1.0 W) laser source expanded through a transmit/receive

Cassegrain telescope. This feature allows the instrument to be operated full-time in

autonomous fashion. (At CRYSTAL-FACE, however, data collection was typically

suspended around solar noon- the lidar siesta time of the tropics- due to the excessive

ambient solar background that adversely affected the photon-counting detector.)

Campbell et al. [2002] have recently summarized the relevant MPL data processing

techniques.

The three participating zenith-pointing Doppler radar systems represent a unique

combination of millimeter-wave to microwave sensors (see Table 1). Atmospheric

probing at the shortest 0.32-cm (94 GHz frequency) wavelength of W-band radars is

sensitive to relatively small cloud droplets and ice crystals, although pulse attenuation in

rain and melting snow can have noticeable effects on the returned signals. The University

of Miami Doppler cloud radar UMDCR [Albrecht et al., 1999] is a single-antenna version

of the radar developed during the early 1980’s [Lhermitte, 1988]. The lightweight

UMDCR uses a high pulse repetition frequency that yields a Doppler velocity window of

±8 ms-1 at high spatial and temporal resolutions (typically 30 m in height by 1-s). With an

antenna beamwidth of 0.24°, the radar horizontal sample size is about 20-m at 5 km. Real

time signal processing is done in with a 14-bit Gage A/D board and PC based CPU. High

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FFT-point Doppler spectra are provided by a real-time FFT algorithm (256, 512 or 1024

FFT points) at all range gates sampled by the processor. The high point FFT gives

excellent Doppler spectra resolution.

At the somewhat longer 0.86-cm wavelength (34.86 GHz frequency) Ka-band

wavelength, the NOAA millimeter cloud radar [MMCR, Moran et al., 1998] shares many

of the capabilities of the W-band radar, but with reduced attenuation effects. The MMCR

provides continuous profiles of the equivalent radar reflectivity factor Ze (mm6 m-3) and

the Doppler spectrum through clouds and precipitation with approximately 10-s temporal

and 45-90 m vertical resolution, using 128 FFT points. In spite of a low peak transmitted

power of 100 W, the MMCR achieves high sensitivity using a large antenna, long

sampling times, and pulse compression techniques.

In contrast to the millimeter-wave radars, the NOAA 10.6-cm (2.835 GHz

frequency) S-band radar cannot generally observe the particles suspended in a cloud, but

rather observes the larger particles that are precipitating out of the cloud. Such radars are

traditional in the sense that pulse attenuation is rarely significant and the returned radar

signals can be treated relatively simply with Rayleigh theory. This vertically pointing

profiler [Ecklund et al., 1999] uses a 3-m parabolic dish antenna and a peak power of 500

W to observe the precipitating particles while they advect overhead. This unit operated

with a 10-s temporal and a 60-m vertical resolution and alternated between an un-coded

and a 10-bit coded pulse compression modes. The wavelength and sensitivity of this

radar limits this radar to resolve particles that are essentially large enough to have a

noticeable terminal fallspeed.

During the early part of the campaign of interest here, a broadening area of low

pressure developed in the south-central Gulf of Mexico, yielding easterly to southeasterly

low-level flow over the research area. In contrast to the usual strong diurnal convective

activity expected during mid-summer months [Michaels, 1985], rainfall at this time was

often more continuous in nature. While not inhibiting strong convective cell

development, more stratiform rainfall events were also encountered. Such conditions are

more amenable for observing radar/lidar melting layer phenomena than in strong

thunderstorms, with their intense vertical motions and highly variable precipitation

conditions.

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3. Data and Analysis

Prior to showing examples from the multiple remote sensor dataset, it is useful to

overview the scattering conditions to be expected at each wavelength in the melting layer,

where various Rayleigh and non-Rayleigh effects will be manifested. Lidar scattering

can be described by the principles of geometric optics, where the exact shape and cross-

sectional area of the ice, mixed-phase, and water particles govern the backscattering

behavior, and the attenuation of the laser pulse will generally be significant. For S-band

radar, it can be assumed that Rayleigh scattering dominates under these conditions, such

that hydrometeor scattering can be treated with spherical and spheroidal dipole particle

models, and is therefore governed by the D6 power law and the particle refractive index

(i.e., phase), while attenuation is unimportant. This corresponds to the traditional radar

bright band scenario. However, at the millimeter W- and K-band radar wavelengths (λ),

a mixture of Rayleigh and non-Rayleigh scattering effects will come into play. For

example, although the size parameter (χ = πD/ λ) for a 5-mm diameter raindrop is 0.15

at λ = 10.6 cm, which lies within the upper limit of χ ≈ 0.3 for Rayleigh (spherical)

particles [Kerker, 1969], the χ ≈ 2.0 and 5.0 for the 0.86- and 0.32-cm wavelengths,

respectively, violate the Rayleigh approximation to an increasing degree. Thus, melting

layer observations at millimeter wavelengths present great challenges to interpret in

comparison to the relative simplicity of traditional radar Rayleigh theory.

A four remote-sensor example of the appearance of the various melting layer

phenomena over a 5-h period on 8 July is given in the height versus time backscattering

displays in Figure 2, where sensor wavelength decreases from top to bottom. Note that a

color radar reflectivity scale in dBZ (10 times log mm6 m-3) is given to the right of each

radar display, and that the range-normalized attenuated lidar backscattering (in arbitrary

units) is also based on a logarithmic scale. Over this period, rainshowers of various

intensity occurred (see bottom panel for surface disdrometer rainfall rate data). The S-

band radar display at top shows a consistent radar bright band centered at ~4.3 km height

above mean sea level (MSL), whose intensity tends to vary with the rainfall rate. The K-

band radar display sometimes indicates a relatively weak bright band at a similar height,

under weak precipitation conditions. Although no evidence for a bright band is apparent

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in the W-band radar display, a radar dark band is sometimes indicated by a decline in

radar signals at ~4.5 km, where dBZ are increasing at the other wavelengths (see below).

The lidar display is quite dissimilar because of the dominating effects of optical

attenuation by hydrometeors, especially in the occasional water clouds below 1.0 km and

from ~3.0 - 4.0 km MSL. Lidar dark bands are easily seen centered just above 4.0 km

from around 1350-1440 and 1615-1720 UTC, which corresponds to periods when the

lidar was able to penetrate high enough to sample the snow causing the rain. Note that

the rapid signal decrease with height in the snow aloft is due to overwhelming

attenuation, which produces a virtual bright band at ~4.5 km, as verified by the radar data

indicating much higher cloud top heights. It is also interesting to note the differences in

the cloud top heights sensed by the S- and W-band radars, which reflect the effects

caused by variable wavelength-dependent attenuation rates versus the λ-4 sensitivity to

particle size that favors ice cloud detection by millimeter-wave radars. Note that although

the K-band radar in this case had a faulty pre-amplifier, which reduced signal levels by

~20 dBZ, the ice cloud top heights are higher than at 10.6 cm in the absence of strong

rainfall-induced attenuation. Particularly near the end of the period, the W-band radar

senses more of the non-precipitating clouds present due to the λ-4 Rayleigh law. These

melting layer features are examined in greater detail below for these and an additional

case study.

Given in Figures 3-5 are comparisons of profiles from the 10-min average MPL

and Doppler radar datasets for three periods on the indicated days showing obvious lidar

dark bands. The closest Miami radiosonde temperature profiles are given at right (the

location of 0°C is highlighted), but it should be acknowledged that precipitation process

can significantly alter the local atmospheric structure and make routine (12-hourly)

sounding data unrepresentative [Steward et al., 1984]. (For example, note the presence

of the isothermal layer in Figure 4 likely caused by the local cooling effect of melting

snowflakes.) The data quantities are attenuated returned power for lidar (in arbitrary

units), and Ze and mean Doppler velocity V for the three radars. Note that the radar dBZ

scale is valid in each case for the S-band data, with the W- and K-band profiles often

adjusted to compress the dynamic range of the total signals and facilitate the data

intercomparison (see figure caption). As indicated in Figure 2, millimeter-wave radar

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returns are often much weaker because of non-Rayleigh scattering effects (i.e., the largest

particles backscatter according to D2 rather than D6), and their radar pulse attenuation

rates can be significant in the melting zone. Thus, we are more interested in the relative

variations in the radar signals than their absolute magnitudes. The profiles bracket the

melting layer from 2.0 to 7.0 km MSL, and shown in each case for reference as the

horizontal green line is the height of the maximum S-band signal in the bright band.

Figure 3 provides a case from the morning of 8 July corresponding to a moderate

rain shower (see Figure 2): the10-min average rainfall rate measured at the ground by the

disdrometer was 1.52 mm h-1. Strong optical attenuation is apparent in the rain and snow,

which contributes to a relatively narrow lidar bright band centered near the 0°C isotherm.

The lidar dark band is broad in this case with two signal minima, and this structure was

persistent with time. The radar profiles show large Ze increases from the top to the

bottom of the melting layer, but the details differ significantly. A radar dark band is

apparent at the top of the melting zone in the W-band data, while a bright band occurs in

the S-band radar data at a lower height. The absence of W- and K-band bright bands

indicates that Rayleigh scattering conditions in the melting snow were violated at these

wavelengths. The Doppler mean velocities also show a steady increase in the melting

layer. However, although the V are similar at ~1.5 m s-1 in the snowfall, the W-band

fallspeeds abruptly stop increasing at ~4.5 m s-1, also due to non-Rayleigh effects. That

is, the K- and S-band radar Doppler V data are weighted toward the largest, fastest-falling

raindrops, which are too large at the 0.32-cm wavelength to behave as Rayleigh

scatterers. The uncommon occurrence of the dual minima in the lidar dark band is

probably a result of the presence of a mixture of ice particle fallstreaks with different size

or type characteristics: note the corresponding broad S-band radar bright band and the

kink in the W-band radar V profile in the lower melting layer.

The two cases in Figures 4 and 5 are from light intensity rain showers (0.07 and

0.09 mm h-1 at the ground, respectively), under which conditions the lidar signals are

obtained from greater heights above the strongly-peaked lidar bright bands owing to the

smaller overall attenuation rates. The increasing lidar and radar signals in the rain with

height above the surface indicate that the rainshowers were in the process of descending,

or that raindrop evaporation in the light rain was occurring. The lidar dark bands in each

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case are rather symmetrical and centered ~100 m below peaks in the S-band radar bright

bands. In Figure 4, from the afternoon of 8 July (see Figure 2), a K-band radar bright

band is clearly indicated, with a peak slightly below the S-band radar bright band center.

Although certainly not a bright band, a barely discernable W-band radar signal peak also

occurs at this height. Furthermore, all three Doppler radar V profiles are in reasonable

agreement, peaking at ~3.5 m s-1. In other words, the particle sizes in the very light rain

in this case did not significantly exceed the Rayleigh limit, and the disdrometer data show

that few drops exceeded 1.0 mm diameter. It is also interesting that the K-band bright

band signal maximum occurs between the heights of the 10.6-cm radar bright band peak

and lidar dark band center, but whether this is a regular feature can not be determined

from our sample generally containing larger rainfall rates.

As in Figure 3, the data from 11 July (Figure 5) again show wavelength-

dependent Doppler velocity differences and the absence of radar bright bands in both the

W- and K-bands. In other words, the absence of a K-band radar bright band indicates that

the larger melting, nonspherical particles violated the Rayleigh scattering assumption,

unlike the raindrops derived from them. A relatively weak W-band radar dark band is

suggested. The lack of radar reflectivity changes with height in the snowfall aloft in this

last case indicates that ice particle aggregation was not important above the melting layer,

in comparison to the small Ze increases seen in the other two cases. As pointed out by

Battan [personal communication, 1978], any radar bright band theory that relies on

processes such as particle aggregation or breakup is not likely to succeed universally.

4. Discussion

The interrelationships between the various optical and microwave melting layer

features illustrated by the characteristic profiles in Figures 3-5 lead us to the conceptual

wavelength-dependent model given in Figure 6. Here we use basic hydrometeor models

to help explain the backscattering phenomena as low-density snowflakes (i.e., dendritic

ice crystal aggregates) transit into homogeneous near-spherical raindrops [Mitra et al.,

1990]. Although we show the position of the 0°C isotherm for reference, the temperature

gradient in the melting layer may be highly variable due to evaporative cooling and other

factors, so we choose not to provide a vertical temperature or height scale. It should also

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be kept in mind that the exact nature of the ice particles undergoing the phase change,

including their density, size distribution, and amount of riming, will affect the

backscattering and velocity outcomes in the melting layer.

At left in Figure 6 is schematically illustrated the hydrometer type, starting at top

with a dry snowflake, two melting snowflakes in which water coatings are accumulating

on the ice crystal branches and inter-branch cavities of the shrinking particles, an

irregular water-enclosed severely melted snowflake, a near-spherical mixed-phase drop, a

drop with most of the ice melted, and finally a homogeneous raindrop. The relative size

of the particles is based roughly on a 10:1 ice to water particle density ratio. These

images are more or less what are actually sensed by lidar, where backscattering responds

to the exact details of particle shape and composition. In contrast, further to the right is a

characterization of the radar cross sections that a S-band radar would sense, where simple

particle models can be employed and the difference in particle phase (i.e., refractive

index) is of significance to backscattering. Such Rayleigh-scattering particle models are

apparently always violated by the larger particles present in rainshowers with W-band

radars, such that a mixture of the optical and microwave models is in effect sensed.

Accordingly, K-band radars with their intermediate wavelengths may sense conditions

somewhere between the S- and W- bands, depending on the sizes of the hydrometeors in

each case. In the remainder of the figure are idealized lidar backscattering (with the

generic effects of attenuation on returned power above the dark band included as the

dashed line), and W- and S-band Ze and V profiles.

First, it should be noted that the strong optical attenuation from snowflakes in the

vicinity of the freezing level, depending on the precipitation rate, helps to create a lidar

bright band. In the absence of aggregation, radar reflectivities and mean fallspeeds are

constant in this region. As the snowflakes progressively melt below the 0°C isotherm

and shrink in size, the returned lidar power steadily decreases and the radar Ze increases

due to the increasing liquid water content. However, we hypothesize that a W-band radar

dark band initially occurs high in the melting region because a water coating on the

largest snowflakes should theoretically generate a decrease in backscattering in these

non-Rayleigh particles [Battan, 1973]. The lidar signals reach a minimum at a stage of

snowflake melting typically corresponding to a position just below the S-band radar

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bright band and even closer to the signal plateau in W-band radar Ze. This leads us to

conclude that the reason the lidar signals start to increase is because the wet snowflakes

have collapsed into mixed-phase raindrops that can now benefit from spherical particle

backscattering mechanisms, namely surface-waves and the front-face axial reflection.

(This collapse happens when the surface tension of the accumulating liquid overwhelms

the structural strength of weakened crystal branches.) Thus, the traditional radar bright

band peak corresponds to Rayleigh scattering in the highly irregular water-coated

particles just prior to mixed-phase raindrop formation. The lidar signal increase below

the dark band center is aided by the removal from the drops center of the embedded ice

mass due to melting and/or internal drop circulations [Pruppacher and Beard, 1970],

which allows the final spherical particle contribution, the paraxial reflection off the far

drop face, to come into play [Ro et al., 1968].

The idealized vertical V profiles in Figure 6 reinforce these inferences, although

at first glance the S-band data would seem to indicate otherwise. Such traditional

Doppler radar data show that particle fallspeeds continue to increase to near the bottoms

of the radar bright and lidar dark bands. However, this position is much lower than the

usual positions of the W-band radar reflectivity plateaus, which must more accurately

demarcate the mean position of the final snowflake to raindrop transition. The W-band

Doppler V profiles in Figures 3 and 5, for example, show that V levels off at a higher

relative position in the melting layer, which also tends to correspond to the millimeter-

wave signal plateaus and the lidar dark band center, than that position at S-band. Thus,

since the mean Doppler velocities in the Rayleigh domain are weighed according to the

D6 power law, microwave radar data are strongly biased toward the few largest particles,

which have fallen the fastest and melted the least.

5. Conclusions

In this study we intended to examine the nature of the lidar dark band using

coordinated Doppler radar measurements in the melting region at three wavelengths

spanning the micro- to millimeter-wave regions (0.32- to 10.6-cm). Presumably, the radar

backscattering features are better understood, and should therefore aid in explaining the

lidar dark band. However, the microphysical/backscattering model that has resulted

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differs from previous models in some respects in both the optical and microwave

domains. Unfortunately, as unique as this dataset is, lidar and radar depolarization data,

which would have provided further information on the state of the melting particles, are

not available from the instruments deployed at the eastern CRYSTAL-FACE field site.

Prior to this study much of what was known of the lidar dark band was restricted

to a single comprehensive case study that established its relation to W-band Doppler

radar data and suggested a likely cause for this melting layer feature [Sassen and Chen,

1995]. Our results confirm that mixed phase particles are indeed involved in both the

creation and destruction of the lidar dark band, with the initial laser signal increase

resulting from the emergence of major spherical particle backscattering mechanisms

immediately after the structural collapse of snowflakes, which is then followed by the

melting of the embedded ice to finally disclose the drop center to backscattering. The

presence and relative location of the triple-wavelength radar melting layer phenomena

here has been crucial in this assessment. The previous lidar dark band model developed

in Sassen and Chen [1995] involves the same basic scenario but over-emphasizes the

contributions of the rear axial backscattering component. Although in laboratory

experiments using frozen pendent drops the particles backscattered ~1.5 to 5.0 times

more energy after the central ice mass floated to the top of the drop [Sassen, 1977b], lidar

dark bands in the field can have more significant overall signal increases. The pendent

particle shape and experimental set-up (using a horizontally incident laser beam) does not

provide the best model for melting snowflakes studied by zenith lidar. In other words,

the frozen drops in the laboratory started out displaying some spherical backscattering

properties, which a melted snowflake would not display. Moreover, the extent that

mixed-phase raindrop nonsphericity affects the lidar melting layer phenomena remains to

be determined. Strong lidar backscattering anisotropy from aerodynamically distorted

raindrops has recently been reported using a scanning lidar [Roy and Bissonnette, 2001].

Comparison of the triple-radar returns in the bright band region reveals significant

wavelength-dependent Ze differences, as well as differences in the basic Doppler

signatures. Because the Doppler V from the three radars are weighted toward different

portions of the particle size distribution, the positions of the snowflake-to-raindrop

transition (i.e., the snowflake structural collapse) appear to diverge as a consequence of

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the violation of Rayleigh theory. The W-band radar measurements consistently failed to

detect the strong backscatter enhancement that we refer to as the radar bright band, only a

gradual Ze increase due to the refractive index consequences of the phase change. K-

band radar bright bands occur under light rainfall conditions, presumably due to the

dielectric constant effect in relatively small (i.e., Rayleigh-scattering) wet snowflakes.

The W-band radar V profiles, least affected by D6 sampling effects, support the

conclusion that the traditional radar bright band occurs immediately above the region that

the severely melted snowflakes collapse into raindrops. In other words, although a water

coating starts accumulating on the ice crystals high in the melting layer to begin

increasing Ze, it is not until a later stage when the melt water collects together within the

particle by capillary action that the peak reflectivities are approached with microwaves.

The subsequent collapse of these decidedly nonspherical mixed-phase particles into

smaller near-spherical drops considerably reduces backscattering with a zenith radar.

As noted first by Lhermitte [1988], W-band radar Ze often decrease in the upper

melting region in another sort of dark band. This radar dark band was attributed to a Mie

backscattering effect in which the mean ice particle size increased enough, perhaps from

riming growth just above the 0°C level, to expose the first backscattering minimum in the

Mie scattering function. (This corresponds to a ~1.0 to 1.6 mm particle diameter increase

for W-band radar.) However, problems with this explanation caused by the widths of the

particle size distribution and other factors were recognized in Lhermitte [2002], and the

possibility that unrepresentative temperature soundings influenced this model should be

considered. We also attribute the radar dark band to non-Rayleigh effects, but not to the

direct Mie effect suggested by Lhermitte. In particular, while a water-coated snowflake

displays increased backscattering in the Rayleigh regime, the opposite is true for larger

particles due to the effects of the different refractive indices of water and ice. We can

refer to this as the melting hail analogy, which causes non-Rayleigh (χ >~2, or D >~

2mm at λ=0.32 cm) ice spheres to backscatter less energy when water-coated than when

dry [Battan, 1973]. Although the concentric water/ice sphere model is not the best

representation for melting snowflakes, it should generally pertain to this feature of the

melting process. Lower in the melting layer, we presume that the shrunken melting

snowflakes behave more like Rayleigh scatterers.

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We emphasize, however, that the exact interrelationships between the various

lidar and radar melting layer features will depend crucially on the precipitation rate, as

well as the size distribution and type of ice particles about to undergo the phase change.

Ice particle density and amount of riming will control the particle fallspeeds, and their

melting rates. Thus, as indicated here, the details of the bright/dark phenomena can vary

noticeably from case to case.

It is interesting that field research is still disclosing new aspects of the effects of

melting layer microphysics on lidar and radar returns. As stated by Lhermitte [2002],

“Even after fifty years of melting layer observations and studies in various parts of the

world, we are still in need of detailed radar observations of reflectivity and Doppler

velocity…using vertically pointing radars working at different wavelengths from 10 cm

to a very short millimeter wave (3.2 mm)”. The research reported here represents a step

in this direction, which also fortunately incorporated the special information from the

vastly different scattering conditions encountered at the 0.523 µm lidar wavelength.

Acknowledgments. This research was supported by NASA grants NAG5-11503,

NAG5-13418, and agreement W-10041 from the CRYSTAL-FACE program, and NASA

grant NAG5-9753 from the TRMM program.

Addresses:

Kenneth Sassen, 903 Koyukuk Drive, Geophysical Institute, University of Alaska

Fairbanks, Fairbanks, AK 99775. email: ksassen@gi.alaska.edu

James R. Campbell, Science Systems and Application, Inc., c/o Code 912, NASA GSFC,

Greenbelt, MD 20771. email: campbell@virl.gsfc.nasa.gov

Pavlos Kollias, Rosenstiel School of Marine and Atmospheric Science, Division of

Meteorology and Physical Oceanography, 4600 Rickenbacker Cswy, University

of Miami, FL 33149. email: pkollias@rsmas.miami.edu

Matthew Shupe, R/ET6, 325 Broadway, Boulder, CO 80305. email:

Matthew.Shupe@noaa.gov

Christopher Williams, NOAA Aeronomy Labatory, 325 Broadway Boulder, CO 80305.

email: cwilliams@al.noaa.gov

15

Jiang Zhu, 903 Koyukuk Drive, Geophysical Institute, University of Alaska Fairbanks,

Fairbanks, AK 99775. email: jzhu@gi.alaska.edu

References

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systems with a MM-wavelength Doppler radar: An Overview. Preprints, 29th

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Atmospheric Radiation Measurement Program Sites: Instruments and data

processing, J. Atmos. Oceanic Technol., 19, 431-442, 2002.

Demoz, B., D. O’C. Starr, D. Whiteman, K. Evans, D. Hlavka, and R. Peravali, Raman

lidar detection of cloud base, Geophys. Res. Lett., 27, 1899-2002, 2000.

Ecklund, W. L., C. R. Williams, P. E. Johnston and K. S. Gage, A 3 GHz profiler for

precipitating cloud studies. J. Atmos. and Oceanic Technol., 16, 309-322, 1999.

Kerker, M., The Scattering of Light and Other Electromagnetic Radiation, Academic

Press, New York, 1969.

Lhermitte, R., Observation of rain at vertical incidence with a 94 GHz Doppler radar: an

insight on Mie scattering, Geophys. Res. Lett., 15, 1125-1128, 1988.

Lhermitte, R., Centimeter and Millimeter Wavelength Radars in Meteorology, Lhermitte

Productions, Miami, 2002.

Meneghini, R., and L. Liao, Effective dielectric constants of mixed-phase hydrometeors,

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Michaels, P.J., Anomalous mid-atmospheric heights and persistent thunderstorm patterns

over Florida, J. Climate, 5, 529-542, 1985.

Mitra, S. K., O. Vohl, M. Ahr, and H. R. Pruppacher, A wind tunnel and theoretical study

of the melting behavior of atmospheric ice particles. VI: Experiment and theory for

snow flakes, J. Atmos. Sci.,47, 584-591, 1990.

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Moran, K. P., B. E. Martner, M. J. Post, R. A. Kropfli, D. C. Welsh, and K. B. Widener,

An unattended cloud-profiling radar for use in climate research, Bull. Amer.

Meteor. Soc., 79, 443-455, 1998.

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and shape of water drops falling at terminal velocity in air, Quart. J. Meteor. Soc.,

96, 247-256, 1970.

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evaporation rates, Appl. Opts., 7, 883-890, 1968.

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on the illumination angle: experimental evidence and geometrical-optics

interpretation, Appl. Opt., 40, 4770-4780, 2001.

Sassen, K., Lidar observations of high plains thunderstorm precipitation, J. Atmos. Sci.,

34, 1444-1457, 1977a.

Sassen, K., Optical backscattering from near-spherical water, ice and mixed phase drops,

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17

Figure Captions

Figure 1. Aerial photograph of the eastern CRYSTAL-FACE field site at the Kendall-

Tamiami Executive Airport near Miami, where clockwise from the top right of the

tarmac are the roof-mounted NOAA W-band Doppler radar, the NASA Surface

Measurements for Atmospheric Radiative Transfer (SMART) van that held the

MPL and several radiometers, the circular rain-guard enclosing the NOAA S-band

radar dish, and the free-standing University of Miami W-band Doppler radar

attached to its supporting van.

Figure 2. Comparison of returned laser energy and triple radar reflectivity factor Ze

height versus time displays over the 1300 to 1800 UTC period on 8 July 2002,

during a series of rainshowers. Gaps in the data records are present for all but the

S-band system. The bottom panel provides the surface rainfall rate measured by

the disdrometer.

Figure 3. From left to right, 10-min average vertical profiles of relative returned laser

power P(R) from the MPL, equivalent radar reflectivity factor Ze (in dBZ) and

mean Doppler velocity V for the three radars (see inserted color key), and

temperature from the closest Miami (MIA) radiosonde, over the indicated time on 8

July 2002. The horizontal green line gives the height of the maximum S-band radar

bright band signal. The range of radar reflectivities has been compressed by adding

22 dBZ and 5 dBZ to the W-band and K-band data, respectively. This signal

manipulation is warranted in view of radar Ze uncertainties caused by non-Rayleigh

scattering effects, and also to some extent by radar calibration and sampling issues

(see Table 1).

Figure 4. As in Figure 3, except that 8 dBZ was added to the W-band radar reflectivity

factors.

Figure 5. As in Figure 3, except that 13 dBZ was added to the W-band radar reflectivity

factors.

Figure 6. A schematic representation of the hydrometeor shapes responsible for the

various lidar and radar bright and dark band features of the melting layer. In the

left two panels are detailed models of melting dendritic snowflakes (i.e., from top

to bottom) that visible-wavelength lidars would sense and the corresponding lidar

18

backscatter coefficients (β), with the effects of laser pulse attenuation shown by the

dashed line. In the middle is a representation of the corresponding models that a S-

band radar would sense. At right are idealized vertical profiles of radar Ze and V

for W- (dashed) and S-band radar. The relative position of the 0°C isotherm is

shown for reference, and the horizontal dotted line corresponds to the lidar dark

band signal minimum.

19

Table 1. Specifications of the micropulse lidar (MPL) and the three radars deployed at the

eastern CRYSTAL-FACE field site. The differences exemplify the wavelength-

dependent range of operational characteristics of modern remote sensors.

MPL W-band K-band S-band

Wavelength 0.523µm 0.32 cm 0.86 cm 10.6 cm

Peak Power (W) 1.0 1000 100 500

Maximum PRF (KHz) 2.5 10 7.7 8.0

Pulse Width 10 ns 200 0.3 µs 0.4 µs

Beamwidth 50 µrad 0.24° 0.3° 3.0°

Receiver Diameter (m) 0.2 0.9 1.8 3.0

Receiver Gain (dBZ) -- 58 165 --

Range Resolution (m) 75 30 45-90 60

Time Resolution (s) 60 60 9-35 10

Range Gates 800 512 184 244

Sensitivity @ 5.0 km -- -37 dBZ -28 dBZ -6 dBZ

V Resolution (cm/s) -- 3.2 6.4 13.9

FIGURE 1

β Ζ V

✶ ✶ ✼ ✽ ❇ ✲ ✻

0°C

✻✴ ✲ ❍ ❍ ❍

●